The double-stranded RNAs of yeast "killer" strains are inherited cytoplasmically and encapsidated in virus-like particles (VLPs). These VLPs resemble in many respects the double-stranded RNA viruses of higher fungi, which in turn are fundamentally simpler versions of the double- stranded RNA viruses of plant and animal cells. The killer VLP is the simplest of these biological entities, in that only a single double- stranded RNA molecule (L, of 2.5 x 10 to the 6th power daltons) is sufficient for capsid synthesis and VLP formation. A second ds RNA molecule (M, of 1.4 x 10 to the 6th power daltons) apparently codes for killer functions. Mutants are available which lack all but a small portion of M. These fragments, the smallest of which (NK3 S) is 0.25 x 10 to the 6th power daltons, are being compared for base sequence homology, in order to map them on their parent RNA. Two such fragments can already be tentatively assigned to one end of M, since they have one 3' end in common and the larger fragment (0.4 x 10 to the 6th power daltons) contains all the T1 oligonucleotides of the smaller fragment. These will be compared with M, in order to determine if they are entirely derived from M, and the 3' ends of M will be sequenced. Other, independently derived, self-replicating fragments of M will also be examined, to determine if they inevitably come from the same end of the parent molecule. The transcription initiation sequence(s) of yeast killer RNAs can also be determined if the killer VLPs have a viral transcriptase, as do animal, plant, and fungal ds RNA viruses.